Fast pulse generator utilizing an electron beam to cause an arc breakdown across thegap region of a coaxial line center conductor



Dec. 26, 1967 Q. A. KERNS 3,360,678

FAST PULSE GENERATOR UTILIZING AN ELECTRON BEAM TO CAUSE AN ARCBREAKDOWN ACROSS THE GAP REGION OF A COAXIAL LINE CENTER CONDUCTOR FiledMay 27, 1965 2 Sheets-Sheet 1 IOO KV POWER SUPPLY Fl LAMENT SUPPLYIINVENTOR.

QUENTIN A. KERNS ATTORNEY.

Dec. 26, 1967 Q. A. KERNS 3,360,678

FAST PULSE GENERATOR UTILIZING AN ELECTRON BEAM TO CAUSE AN ARCBREAKDOWN ACROSS THE GAP REGION OF A COAXIAL LINE CENTER CONDUCTOR FiledMay 27, 1965 2 Sheets-Sheet 2 V 0 a t \1 i (1) m5 (9 2] r C? M Q F3 g303 1O I: g L ammy f INVENTOR. \1 QUENTIN A. KERNS ATTORNEY.

United States Patent FAST PULSE GENERATOR UTILIZING AN ELEC- TRON BEAMTO CAUSE AN ARC BREAKDOWN ACROSS THE GAP REGION OF A COAXIAL LINE CENTERCONDUCTOR Quentin A. Kerns, Orinda, Calif., assignor to the UnitedStates of America as represented by the United States Atomic EnergyCommission Filed May 27, 1965, Ser. No. 459,484 12 Claims. (Cl. 315-3)The present invention relates generally to an electrical pulse generatorand more particularly to apparatus for producing short electrical pulsescharacterized by very rapid rise and decay times.

Very fast pulses with a length in the order of a single nanosecond (1l() second) are difficult to produce, particularly if very rapid riseand decay times are necessary. Such pulses are useful for controllablysimulating the type of pulse produced by nuclear charged particles andtherefore are useful for predicting the response of nuclear particledetectors to charged particles. Fast pulses are also useful in highprecision timing apparatus where it is further necessary that the timeat which the pulse is produced be very accurately controllable, withminimum delay between the application of a trigger impulse and theproduction of the switching pulse.

In the present invention, pulses are generated by cre ating asteady-state electrical charge between the inner and outer conductors ina short section of a coaxial transmission line. When the charge in theshort line section is suddenly discharged, a pulse is created. Thus, togenerate the desired type of pulse, it is necessary to provide a switchin the coaxial line for removinng the charge on the center conductor.However, if high frequency signals or fast pulses are being carried in acoaxial line, the size and spacing of the inside and outside conductorsshould be constant, that is, the characteristic line impedance should beconstant along the entire length of the line. Physical irregularities ina line as created by ordinary switches may create electricalperturbations which cause signal reflection with consequent loss ofsignal power and create deleterious spurious pulses. Therefore, when aswitch is provided in a coaxial line, the switching mechanism should notaffect the diameter of the central conductor nor the spacing between thecentral conductor and the outside shield conductor of the coaxial line.Unfortunately, the mechanism of conventional switches occupiesconsiderable space within the cable and necessitates altering the cabledimensions. Furthermore, the operation of most conventional switchescannot be accurately timed in terms of a microsecond or less.

The present invention provides for a very rapid switch action at thecentral conductor of a coaxial line, the switching elements causingminimal perturbation of the line impedance. In the invention, aminiature electron gun disposed outside the outer conductor of the linedirects an accelerated beam of electrons into the coaxial cable at aswitch gap, one side of which is the charged section of the centralconductor. The electron beam causes an arc breakdown across the gap andthus discharges the line. Ordinarily, such a switching means isimpractical since the space between the inner and outer conductors of acoaxial line must contain air or some other gas since breakdown acrossthe switch gap must occur through a gas. In any practical apparatus, abeam of electrons can be accelerated only in a vacuum: In the presentinvention, the electron beam is created in a vacuum tube and passedthrough a thin electron transparent vacuum sealing-window in the outerconductor and thus into the gaseous atmosphere around the switch p- Itis an object of the present invention to provide an improved pulsegenerator producing very rapid pulses with fast rise and decay times.

It is an object of the present invention to provide an improved pulsegenerator by utilizing very fast action switching action in a coaxialtransmission line.

It is another object of the present invention to provide an improvedpulse generator having a coaxial line switch causing minimal change inthe characteristic impedance of the line.

It is another object of the present invention to provide an improvedpulse generator utilizing a coaxial line switch having no moving partsand capable of being remotely controlled.

It is another object of the present invention to provide a pulsegenerator capable of an output pulse immediately after receipt of aninput trigger pulse.

The invention will be better understood by reference to the accompanyindrawing of which:

FIGURE 1 is a broken-out view of a fast pulse generator including acoaxial line with which an electron gun is provided for efiectingswitching, and FIGURE 2 is a broken-out view of another embodiment ofthe fast pulse generator.

Referring now to FIGURE 1, there is shown a coaxial transmission line 11having a round outer shield conductor 12 and an inner center conductor13 disposed along the axis of the shield. Typically, the centerconductor 13 is supported by a plurality of insulative cones 14 disposedat intervals inside the shield 12, each cone having a central aperturein which the center conductor is supported.

First and second sections 15 and 20 of center conductor 13 areelectrically separated by a switch gap 16 provided in the centerconductor. The switch gap 16 is between spaced apart overlapping ends 17and 18 of the conductor 13, the ends being flattened on each side of thegap. The ends 17 and 18 are thus electrodes of a switch gap 16 acrosswhich an arc is created to obtain the switch function. End 18 ismultiapertured so that an electron beam can be directed throughapertures 19 and across the gap 16.

A power supply 21 has a negative terminal connected to the outer shield12 while a positive terminal is connected through a current limitingresistor 22 to the first section 15 of center conductor 13. The outputend of second section 20, that is, the end of section 20 opposite end18, is terminated in the characteristic impedance of the coaxial line11. Such impedance is indicated in FIGURE 1 as a resistor 23 connectedfrom the shield 12 to the center conductor 13. When the switch is open,that is, there is no arc across the gap 16, the full potential of powersupply 21 is present across the gap. Thus the voltage potential ofsupply 21 must be sulficiently low that self-triggering breakdown doesnot occur across the gap 16.

Controlled triggering of an arc across the gap 16 is obtained byinjecting a beam of electrons through the apertures 19 to ionize the gasin the gap so that a low resistivity current path is created from end 18to end 17. Such electron beam is created in a special vacuum tube 29 inwhich there is an insulative cylindrical evacuated tube envelope 31having at one end a conventional control grid 32, cathode 33, and heaterstructure 34, providing a beam of electrons directed toward a flatcircular anode 36 which closes the opposite end of the envelope. Anode36 is typically a one mil thick beryllium or nickel diaphragm.

A plurality of annular electron accelerating electrodes 37 are coaxiallymounted in the wall of tube 29, between the grid 32 and the anode 36, apotential difference being provided between each electrode 37 toaccelerate electrons emitted from the cathode 33 toward the anode 36. Toprevent damage to the envelope 31 and to prevent deposits from beingformed on the envelope as a result of sputtering or arcing between theelectrodes 37, each electrode has a flared configuration to shield theenvelope, the smaller diameter ends of the electrodes being directedtowards anode 36. For similar reasons, the anode 36, which is thin andfragile, is protected from being ruptured by arcs by a cylindricalshield ring 38, which is disposed coaxially with the anode at the innersurface thereof so that any arcs will strike such ring in preference tothe anode 36. A many apertured circular outer shield 39 is disposed atthe end of tube 29 against anode 36 for protecting the anode fromexternal physical and electrical damage. A similar apertured shield 41is disposed against the inside surface of the anode 36 to prevent airpressure from bulging the anode inwardly. The apertures of inside shield41, outer shield 39, and end 18 of the center conductor 13 are alignedto facilitate the passage of the electrons therethrough.

The electrical potentials for the electrodes 37 of tube 29 are obtainedfrom a high voltage power supply 46 having a potential of approximately100 kilovolts. A voltage divider 47 comrpised of a plurality of seriesconnected resistors 48 is connected across the power supply 46. Each ofthe electrodes 37 is electrically connected at an intermediate pointalong the divider so that the electrical potential of the electrodes isprogressively more negative going from the anode 36 toward the controlgrid 32. A filament power supply 49 provides heating current to thefilament 34 for heating the cathode 33. A bias voltage supply 51 has anegative terminal connected to the negative terminal of the high voltagepower supply 46. The cathode 33 is connected to the positive terminal ofbias supply 51 through a cathode resistor 53 so that a cut-off bias isnormally present between cathode 33 and grid 32, that is, normally noelectrons pass through the control grid.

If a negative voltage pulse is applied to a control terminal '2,connected to the cathode 33, the negative grid bias potential isovercome and electrons from the cathode 33 pass through the control grid32 and are accelerated toward anode 36 by electric fields between theelectrodes 37. In operation, some of the electrons are lost by collisionwith the apertured shield 41, but most of the electrons pass through theapertures, striking the anode 36 with sufficient velocity to passthrough, and emerging from the apertures in shield ring 39. Since theconductor section 20 is at the same potential as the outer conductor 12but is negative relative to the conductor section 15, electrons from thetube 29 are accelerated through the apertures 19 to the end 17 andionize the gas between the ends 17 and 18, causing breakdown across thegap 16. Because of the low impedance of the are, the effect is similarto closing a switch across gap 16.

Before the arc across gap 16 is created, an electric field is providedbetween the first section 15 of the center conductor of the coaxial lineand the outer conductor 12 by the power supply 21. With the formation ofan arc across the gap 16, the energy in the electric field is suddenlyapplied to the remainder of the coaxial line and a very sudden steepwavefront pulse is propagated down the line 11, appearing across theterminating resistor 23. The relatively high impedance of resistor 22,compared to the impedance of the arc, effectively decouples the powersupply 21 from the conductor section 15 during the pulse formingprocess. Both the rise and decay time of a pulse are very short whilethe duration of the pulse is equal to the time it takes anelectromagnetic wave to traverse the coaxial line from the gap 16 to theopen end of first coaxial line section 15 and back.

The physical arrangement of certain elements of the invention may bevaried, another configuration being shown in FIGURE 2. The beamaccelerating vacuum tube 29 in FIGURE 2 is identical to that describedwith regard to FIGURE 1 and is therefore not shown in detail, but theconstruction of the coaxial line 61 differs in that the outer shieldconductor 62 has apertures 59 in an end plate 63 through which theelectron beam from tube 29 is directed. The central conductor 64 of thecoaxial line 61 has an end section 66 of reduced diameter adjacent theend plate 63, the section 66 being covered with insulation 67 over whicha metal sleeve 68 is disposed. The sleeve 68 has the same diameter asthe central conductor 64 and forms one plate of a capacitor, the otherplate being the section 66. The end 69 of sleeve 68 faces the apertures59, the end 69 and plate 63 being electrodes of a spark gap 71. A powersupply 72 has a positive terminal coupled through an isolating resistor73 to the sleeve 68 while the negative terminal is connected to theouter shield 62. The coaxial line 61 is terminated in the characteristicimpedance of the line, as represented by a resistor 74.

In operation, an electrostatic field is established between the sleeve68 and shield 62 by the power supply 72. A trigger pulse applied to thetube 29 causes a beam of electrons to pass through the apertures 59 andstrike the end 69, ionizing the gas therebetween. An are immediatelyforms between the end 69 and the end 63, the low resistance of the areeffectively connecting the sleeve 63 to the end 63. The capacitancebetween the sleeve 68 and the center conductor section 66 causes a pulseto be applied to the center conductor, the length of the pulse beingtwice the time it takes an electromagnetic wave to travel the length ofthe sleeve 68, the wave going from the end plate 69 to the opposite endof the sleeve 68 and returning.

Such fast pulses are useful in the evaluation of fast pulse circuits,particularly for controllably simulating the type of pulse created bynuclear radiation in detection instruments. When the pulse generator isutilized in a timing device it is important that the pulse be formedimmediately upon application of a trigger pulse to the control terminal52. In the present invention the delay between the input and outputpulses is typically five nanoseconds (5-10- seconds) or less.

While the invention has been disclosed with respect to particularembodiments, it will be apparent to those skilled in the art thatnumerous variations and modifications may be made within the spirit andscope of the invention and thus it is not intended to limit theinvention except as defined in the following claims.

What is claimed is:

1. In a fast pulse generator, the combination comprising a coaxialtransmission line having an outer and an inner conductor, a firstportion of said inner conductor being spaced from the outer conductorand from the remainder of the inner conductor of said line, meansproducing an electrostatic field between said first portion of saidinner conductor and said outer conductor of said coaxial line, and apulsed electron beam generator directing energized electrons betweensaid first portion of said inner conductor and an adjacent one of saidconductors to effect discharge of said electrostatic field.

2. In a fast pulse generator, the combination comprising a coaxialtransmission line having an inner conductor and an outer shieldconductor, said inner conductor being electrically separated into firstand second portions, a direct current power supply providing a charge onsaid first portion of inner conductor relative to said shield conductor,an electron gun of the class having an electron emitting cathode and acontrol electrode and an anode which is adapted to pass electronstherethrough, said electron gun having electron accelerating meansdisposed between said anode and said control'electrode, said anode beingpositioned to pass said electrons into a region within said shieldconductor and proximal to said first portion of said inner conductor toinitiate discharge thereof.

3. The fast pulse generator as described in claim 2, wherein saidelectron gun is further comprised of a plurality of progressively morepositive coaxial annuli spaced between said control electrode and saidanode and each having a center aperture for said electron beam.

4. In a fast pulse generator the combination comprising a coaxialtransmission line of the class having a cylindrical outer conductor anda spaced apart inner conductor extending along the axis thereof, saidinner conductor being divided into first and second axially spacedsections with said first section being separated from an adjacent one ofsaid conductors by a discharge gap, said outer conductor having anaperture therein facing said discharge gap, a source of electricalpotential connected across said outer conductor and said first sectionof inner conductor, and an electron accelerating tube disposed adjacentsaid outer conductor and having the electron emitting end thereofdirected into said aperture and towards said discharge gap, saidelectron accelerating tube having a cathode opposite said first end andan electron transmitting anode at said first end.

5. A fast pulse generator as described in claim 4, wherein said anode ofsaid electron accelerating tube is comprised of a thin foil disposedacross said first end of said tube, and at least one relatively thickstiffener disposed across said first end of said tube against said foil,said stiffener having a plurality of distributed transverse passagestherethrough for transmitting electrons to said foil.

6. In a fast pulse generator, the combination comprising a coaxialtransmission line having an outer shield conductor and a centralconductor, said central conductor of said transmission line having a gaptherein, a power supply coupled across said gap and creating a potentialdifferential thereacross and having a voltage less than a spontaneousspark breakdown value, an opening in said outer conductor adjacent saidgap, and an electron gun disposed outside said coaxial line and havingan electron discharging end directed through said opening and acrosssaid gap.

7. In a fast pulse generator the combination comprising a coaxialtransmission line having an outer shield conductor and an innerconductor, said inner conductor being divided into a first and a secondsection each having first and second ends, said first end of said firstsection being spaced from said second end of said second section andproviding a spark gap therebetween, said second end of said secondsection having apertures therethrough, a direct current power supplyhaving a positive terminal connected to said first section of said innerconductor and having a negative terminal connected to said shieldconductor, and an electron gun having an electron discharge end directedinto said gap through said apertures in said second end of said secondsection of inner conductor.

8. A fast pulse generator as described in claim 7, wherein said firstend of said first section of inner conductor overlaps said second end ofsaid second section thereof along the axis of said coaxial line, saidoverlapping ends being flattened and being spaced apart in thetransverse direction with respect to said axis.

9. In a fast pulse generator, the combination comprising a coaxialtransmission line having an inner conductor and an outer shieldconductor, an apertured conductive end plate disposed across an end ofsaid shield conductor, a tubular sleeve disposed coaxially within saidouter shield and adjacent said end plate, said sleeve being colinearwith said inner conductor, means capacitively coupling said sleeve tosaid inner conductor, an electron beam generator directed through saidapertured end plate to said sleeve, and a power supply having a positiveterminal coupled to said sleeve and a negative terminal coupled to saidshield conductor.

10. A fast pulse generator as described in claim 9, wherein one endsection of said inner conductor has a reduced diameter and extendscoaxially into said sleeve to form said capacitive couplingtherebetween, a dielectric element being provided between said endsection and said sleeve.

11. A fast pulse generator as described in claim 9, wherein a resistoris connected between said sleeve and the positive terminal of said powersupply.

12. A fast pulse generator as described in claim 9, wherein saidelectron beam generator is comprised of an insulative cylindricalenvelope, an electron emitting cathode disposed at a first end of saidenvelope, an electron accelerating means in which a plurality of spacedapart annular electrodes are disposed coaxially in said envelope atintervals along the path of said electron beam between said cathode anda second end of said envelope, a power supply providing progressivelyhigher positive electrical potentials to said electrodes from saidcathode to said second end of said envelope, a thin diaphragm disposedacross said second end of said envelope, an apertured disc disposedagainst the outwardly facing surface of said diaphragm, and an annularprotective electrode disposed coaxially within said envelope andprojecting from said diaphragm toward said cathode.

References Cited UNITED STATES PATENTS 2,709,229 5/1955 Linder 315-3 X3,319,114 5/1967 Larson 33397 X HERMAN KARL SAALBACH, Primary Examiner.P. L. GENSLER, Assistant Examiner.

1. IN A FAST PULSE GENERATOR, THE COMBINATION COMPRISING A COAXIALTRANSMISSION LINE HAVING AN OUTER AND AN INNER CONDUCTOR, A FIRSTPORTION OF SAID INNER CONDUCTOR BEING SPACED FROM THE OUTER CONDUCTORAND FROM THE REMAINDER OF THE INNER CONDUCTOR OF SAID LINE, MEANSPRODUCING AN ELECTROSTATIC FIELD BETWEEN SAID FIRST PORTION OF SAIDINNER CONDUCTOR AND SAID OUTER CONDUCTOR OF SAID COAXIAL LINE, AND APULSED ELECTRON BEAM GENERATOR DIRECTING ENERGIZED ELECTRONS BETWEENSAID FIRST PORTION OF SAID INNER CONDUCTOR AND AN ADJACENT ONE OF SAIDCONNDUCTORS TO EFFECT DISCHARGE OF SAID ELECTROSTATIC FIELD.